The subject matter disclosed herein relates to an X-ray CT (Computed Tomography) apparatus for imaging a subject by helical scanning.
Conventionally, X-ray CT imaging on a heart, especially, coronary arteries or the like, of a subject to be imaged involves so-called electrocardiographic gating imaging in which imaging is performed in synchronization with heartbeats.
The electrocardiographic gating imaging includes a prospective imaging method and a retrospective imaging method. The prospective imaging method comprises observing a cardiac cycle averaged based on a plurality of latest heartbeats, and collecting projection data for image reconstruction in synchronization with a cardiac phase at which motion of the heart is considered to be slowest, for example, at a 75% phase of the averaged cardiac cycle (see Japanese Patent Application Laid Open No. 2006-006531, for example). The retrospective imaging method comprises imaging a subject by a helical scan with a relatively small helical pitch, for example, with a pitch of the order of 0.2, while monitoring electrocardiographic signals from the subject, to collect the electrocardiographic signals for the subject and projection data for the subject simultaneously, and later extracting only projection data corresponding to an arbitrary cardiac phase for image reconstruction (see Japanese Patent Application Laid Open No. 2003-164446, for example).
Another method for reconstructing a tomographic image of coronary arteries or the like in a heart is known as a so-called multi-segment image reconstruction method. The method comprises collecting projection data in a plurality of segments at the same cardiac phase over several heartbeats, or extracting such projection data from those continuously collected in a helical scan, combining the projection data over view angles required in reconstruction of an image for one slice, and reconstructing a tomographic image. In general, reconstruction of an image for one slice requires projection data over a view angle of at least an X-ray beam fan angle plus 180°, and in a case that projection data collected during one rotation of a gantry cannot cover the view angle, a tomographic image for one slice can be reconstructed using the multi-segment image reconstruction method by combining segmented projection data collected in a period of time corresponding to a plurality of consecutive heartbeats.
Such electrocardiographic gating imaging or multi-segment image reconstruction enables image reconstruction processing to be achieved using projection data corresponding to a given cardiac phase zone in which motion of the heart is slow, so that a tomographic image of a heart in which blurring of a subject due to cardiac motion is suppressed can be obtained.